Currently available dental materials for the fabrication of provisional restoratives include polymerizable resins, which have replaced the use of metal shells and polycarbonate temporary crowns. Resin-based crowns can be made available in different shades that can be matched to the patient's dentition. Two-part resin formulations are typically used wherein the resin constituents are separate from the initiator system. The resin constituents are typically free radically polymerizable monomers (e.g., acrylates, methacrylates, and acrylamides). The initiator system typically includes reducing agents, such as tertiary amines, and oxidizing agents, such as peroxides, as accelerators or activators for free radical polymerization and control of the rate of polymerization.
It is desirable to control the rate of polymerization such that the dental material has a relatively rapid initial set phase time during which time a specific hardness is attained without locking into the undercuts produced during tooth preparation, followed by an extended flexible phase work time. During this flexible phase the dental material can be removed from the mouth to allow the dental practitioner time to modify, trim, and adjust the dental material for a custom fit onto the prepared tooth. Modification and adjustment of the dental material during the extended flexible phase is desirable in order to provide a more custom fit of the provisional restorative in the mouth of the patient. Additionally, heat generated by the exothermic polymerization is controlled by removal of the dental material from the intra-oral environment. This also avoids damage to oral tissues and consequently provides more comfort for the patient. Before final placement in the mouth and cementing in place, the restorative is completely hardened by conventional means commonly known for free radical polymerization.
A typical procedure for making a provisional dental restorative involves the following steps. Initially, an alginate impression is taken before preparing the teeth. The impression is rinsed, set aside, and wrapped in a moist paper towel. The teeth are then prepared and the correct shade of acrylic powder is selected to match the natural teeth. An acrylic liquid resin and the acrylic polymeric powder, one of which includes a reducing agent and the other of which includes an oxidizing agent, are mixed together and placed in the impression. The impression is placed aside until the composition thickens and forms a dull appearance (approximately 45-60 seconds). Meanwhile, the prepared teeth and surrounding tissue are coated with a petroleum jelly, which ensures easy removal of the acrylic temporary from the preparation and protects the teeth and tissue from irritation by the acrylic mixture. The impression with the acrylic mixture is seated in the mouth and held in place for a sufficient time to allow it to harden to a removable state. Mixing through this stage is the initial set phase time. The acrylic material is removed from the impression and gross excess acrylic is trimmed. The acrylic material is placed in and out of the mouth while the acrylic material is in a rubbery state. This is the extended flexible phase work time. The acrylic material is removed from the mouth and set aside until the acrylic is fully cured. The fit of the acrylic restorative is checked and adapted to fit, if necessary. Excess acrylic is trimmed with an acrylic bur or stone and polished to a smooth finish. The acrylic temporary is then cemented into place.
The important time during the preparation of a restorative occurs after the resin, initiator, and reducing agent are mixed together. It is important that hardening occurs initially rapidly (during the initial set phase time), but then slows to allow a dental practitioner to place, remove, and trim the restorative material (during the extended flexible phase work time) before it continues to harden to such a state as to become unworkable. There is a need for dental materials that provide reasonable initial set phase times and flexible phase work times that are more readily controllable than conventional materials.